1. Field of the Invention
The invention relates to an image forming apparatus.
2. Description of the Related Art
In connection with an image forming apparatus, a color-matching adjusting process is known which is performed to correct a shift of a printing position of each color ink on a recording sheet. In the color-matching adjusting process, quality checkup images are actually printed on a recording sheet with respective color inks, the overlapping state of the images of the respective colors is detected, and a printing position of each color is corrected based on the detection result. The image forming apparatus performs the color-matching adjusting process when a degree of shift of the printing position of each color is anticipated to be outside a predetermined proper range in the quality checkup image.
Examples of the case where a degree of shift of the printing position of each color is anticipated to be outside a proper range include a case where a predetermined time has passed, a case where a predetermined number of images have been formed, a case where the environment has changed, and a case where a power supply has been turned on or off, after a previous adjusting process was performed.
Moreover, for example, Japanese Patent Application Laid-open No. 2004-117384 discloses a technique for use in an image forming apparatus, which predicts whether a degree of shift of a printing position of each color is outside a proper range, based on usage conditions or environment (e.g., temperature) at the time of performing color-matching adjustment for the purpose of suppressing waste of a developer or the like and executing the color-matching adjustment with high efficiency.
However, in an image forming apparatus including a plurality of writing units, the conventional color-matching adjusting process has a problem in that a color shift occurs easily at the time of switching from a monochrome printing operation to a color printing operation. The color shift may be resolved by frequently performing the color-matching adjustment. However, such a technique accompanies another problem of an increased downtime because a printing process cannot be performed during the color shift adjustment.
Therefore, there is a need for an image forming apparatus that is capable of decreasing a color shift in a color printing, and reducing downtime.